The general objective of this project is to investigate further the structure and function of plasma and platelet factor XIII. Factor XIII is the proenzyme which, in its activated state, participates in the terminal phase of the coagulation of blood and catalyzes the chemical crosslinking of fibrin. Development of radioimmunoassays for the subunits of the zymogen and for the active enzyme are an important part of this project. Thus far a sensitive RIA has been established for the b subunit. This has been used to quantitate b subunit in normal persons, 13 factor XIII deficient patients, their kindred, and three patients with antibody inhibitors to factor XIII. Preliminary data indicate an interdependent effect on the concentrations of a and b subunits. Specific research goals for the proposed work include further studies with the RIA for b subunit and development of an RIA for a subunit. Studies will also be conducted on a family with congenital factor XIII deficiency which appears to have an abnormal subunit. In further studies the effect of plasmin proteolysis of the different molecular forms of factor XIII will be investigated.